To do this project, you will need access to a laboratory with facilities for culturing bacteria. You should be familiar with sterile technique and proper handling of bacterial cultures.

Material Availability

Specialty items

Cost

High ($100 - $150)

Safety

Standard precautions for handling bacterial cultures and bleach.

Abstract

How does your family thaw and cook meat? Have you ever wondered if it is the safest way? In this practical science project, you can find out and shed light on safe practices in the kitchen by investigating how many viable bacteria are present in samples of meat that have been thawed or cooked using different methods.

Objective

To determine which method of defrosting meat is safest and which method of cooking kills the most bacteria.

APA Style

Science Buddies Staff.
(2014, December 6).
Minimizing Bacteria in the Thawing and Cooking of Meat.
Retrieved December 9, 2016
from http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p015.shtml

Share your story with Science Buddies!

Introduction

Which is the safer way to thaw frozen meat: at room temperature or in the refrigerator? For cooking meat safely, is a microwave as good as a conventional oven?

One way to find out is to measure how many viable bacteria are present in samples of meat that have been thawed or cooked by the different methods mentioned above. How do you measure the number of viable bacteria? One way is to homogenize a sample of meat in a blender, dilute the sample, and then plate it on a bacterial culture plate. The plate is then incubated overnight (or longer), and visible colonies of bacteria are then counted. The goal is to dilute the sample sufficiently so that individual bacteria are separated from one another on the plate, meaning that each colony will have arisen from an individual bacteria—referred to as a colony forming unit or CFU.

Typical laboratory cultures have between 106 and 109 bacteria/mL, and for plating bacteria, you typically use a volume of 100 μL (which is the same as 0.1 mL). So if you simply took your sample straight from the culture, you'd expect to have between 105 and 108 (100,000 to 100,000,000) bacteria in your 100 μL sample. Obviously, you would end up with far too many colonies to count! In fact, the plate would be so densely covered that you wouldn't be able to distinguish individual colonies.

To get around this problem, the obvious solution is to dilute the sample. If you wanted to end up counting about 100 colonies per plate, then you'd need to dilute between 1,000– and 1,000,000-fold. It's not practical to make such large dilutions in a single step, so a good way to do this is by using serial dilutions. The diagram in Figure 1, below, illustrates the process. Each tube starts out with 9 mL of sterile water. 1 mL of the bacterial culture solution is added to the first tube, and mixed. This dilutes the bacterial solution by a factor of 10. Then, 1 mL of the solution from the first tube is removed, and added to the 9 mL of sterile water in the second tube. This is another 10-fold dilution, making a 100-fold dilution of the original solution.

Figure 1. Serial 10-fold dilutions of a bacterial sample.

The serial dilutions are continued in a similar manner until the desired final dilution is achieved. In order to calculate how many bacteria were present in the original solution, you count colonies on the plate, and then multiply by the total dilution factor. You can see that it is important to make the volume measurements accurately and reproducibly for this process. Errors in measurement will cause errors in the bacterial count.

In this project, you'll see how you can apply this method to figure out how many viable bacteria there are in samples of meat that have been exposed to different thawing and cooking conditions.

Terms and Concepts

To do this project, you should do research that enables you to
understand the following terms and concepts:

colony forming unit (CFU),

serial dilution.

Questions

Why are the samples diluted so many times before plating them on agar?

What are some potential sources for error in the bacterial counts in this experiment?

What can you do to minimize those sources of error?

For more advanced students, what can you do to estimate the size of the error in the bacterial counts? (See Variation 1, below.)

Bibliography

The first part of this webpage describes the method used in this project for estimating the number of bacteria in samples. More advanced students can read the entire page and learn about chi-square curve-fitting:
Kirkman, T., date unknown. "Chi-Square Curve Fitting," Department of Physics, College of St. Benedict & St. John's University [accessed September 14, 2006] http://www.physics.csbsju.edu/stats/chi_fit.html.

This webpage has background information on bacterial growth. It is from an online textbook of bacteriology, which can be an excellent source of further information on bacteria:
Todar, K., 2002. "Growth of Bacterial Populations," Todar's Online Textbook of Bacteriology, Department of Bacteriology, University of Wisconsin, Madison [accessed September 14, 2006] http://textbookofbacteriology.net/growth.html.

News Feed on This Topic

,
,

Note: A computerized matching algorithm suggests the above articles. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more about the News Feed

Materials and Equipment

Note: If you are carrying out this experiment in a school laboratory, some of the materials and equipment listed below may be more readily accessible.

Disclaimer:
Science Buddies occasionally provides information (such as part numbers, supplier
names, and supplier weblinks) to assist our users
in locating specialty items for individual projects. The
information is provided solely as a convenience to our users. We do our best to make sure that part numbers
and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted
or improved, please send us an email if you run across any parts that are no longer available.
We also do our best to make sure that any listed supplier provides prompt, courteous service.
Science Buddies does participate in affiliate programs with
Amazon.com,
Carolina Biological, Jameco Electronics,
and AquaPhoenix Education.
Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity. If you have any comments (positive or negative) related to
purchases you've made for science fair projects from recommendations on our site, please let us know. Write
to us at scibuddy@sciencebuddies.org.

Share your story with Science Buddies!

Experimental Procedure

Working with Biological Agents

For health and safety reasons, science fairs regulate what kinds of biological materials
can be used in science fair projects. You should check with your science fair's
Scientific Review Committee before starting this experiment to make sure your science
fair project complies with all local rules. Many science fairs follow Intel®
International Science and Engineering Fair (ISEF) regulations. For more information,
visit these Science Buddies pages:
Projects Involving Potentially Hazardous Biological Agents and
Scientific Review Committee. You can also visit the webpage
ISEF Rules & Guidelines directly.

Do the bacterial counts as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section below.

Divide the remaining meat pieces into two equal piles, wrap each in plastic wrap & foil, and freeze (up to 14 days).

Thaw one package of meat pieces at room temperature until the internal temperature of meat the reaches room temperature. Record how long this takes and the room temperature in your lab notebook.

Create a data table in your lab notebook to record this data, and other data you collect, while doing your experiment.

Thaw the other package of meat pieces in the refrigerator until soft. Record how long this takes and the temperature in your lab notebook.

Measure the bacterial count for two samples from each condition.

Do the bacterial counts as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Test 2 - Cooking Meat in Microwave

Take two samples of meat thawed at room temperature and two samples thawed in the refrigerator and cook each of them separately in the microwave until the internal temperature of each meat sample reaches the recommended temperature: beef, 72°C; pork, 77°C; chicken, 85°C.

Measure the bacterial counts of the cooked meat samples as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Test 3 - Cooking Meat in Standard Oven

Take two samples of meat thawed at room temperature and two samples thawed in the refrigerator and cook them each separately using a standard oven heated to 190°C (375°F). Recommended cooking times are: beef, 20 min.; pork, 17 min.; chicken, 15 min.

Measure the bacterial counts of the cooked meat samples as described in the "Measuring the Number of Bacteria Present in a Meat Sample" section, below.

Measuring the Number of Bacteria Present in a Meat Sample

Puree a piece of meat (sample) in a sterilized blender with a small amount of sterile water (use the same amount of water each time).

Note: To sterilize the blender, clean the jar and blade assembly with warm, soapy water and allow to dry. Wrap the blender jar and blade assembly only (not the base with the electric motor) in aluminum foil, and heat in an oven at 250°F for 30 minutes. Use oven mitts when removing it from the oven. Unwrap the foil just before use.

Using sterile technique, do 10-fold serial dilutions (9–11 times) of the pureed meat sample and then plate a 100 μL sample. You will have to determine how many times to dilute. We suggest using the sterile pipettes, a sterile 10 mL graduated cylinder, and sterile 15 mL test tubes for preparing the dilutions, and then plating 100 uL of solution from each of the last three dilutions (on three separate tryptic soy agar plates) to see which dilution works best for counting colonies.

Incubate the plates, inverted (agar on top), at 37°C, overnight or longer. Use a longer incubation time if necessary (for example, for incubation at lower temperature). Once you have determined the procedure (serial dilution and incubation time) that works best, stick with it for all of your counts.

Divide the plate into 4 quadrants, and use the stereo microscope to count colonies in one quadrant. Multiply count by 4 to get total CFU/100 μL for the diluted sample. Then multiply by the total dilution factor to get CFU for each piece of pureed meat. You can then divide by the original weight of the meat to get CFU/g of meat.

Bacterial Safety

Bacteria are all around us in our daily lives and the vast majority of them are
not harmful. However, for maximum safety, all bacterial cultures should always be
treated as potential hazards. This means that proper handling, cleanup, and disposal
are necessary. Below are a few important safety reminders. You can also see the
Microorganisms Safety
Guide for more details. Additionally, many science fairs follow
ISEF Rules & Guidelines, which have specific guidelines on how bacteria
and other microorganisms should be handled and disposed of.

Keep your nose and mouth away from tubes, pipettes, or other tools that come in
contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.

Make sure to wash your hands thoroughly after handling bacteria.

Proper Disposal of Bacterial Cultures

Bacterial cultures, plates, and disposables that are used to manipulate the bacteria
should be soaked in a 10% bleach solution (1 part bleach to 9 parts water)
for 1–2 hours.

Use caution when handling the bleach, as it can ruin your clothes if spilled, and
any disinfectant can be harmful if splashed in your eyes.

After bleach treatment is completed, these items can be placed in your normal household
garbage.

Cleaning Your Work Area

At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach
solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly
clean any surfaces you have used.

Be aware of the possible hazards of disinfectants and use them carefully.

Share your story with Science Buddies!

Variations

For a more advanced project, you should try to determine the reliability of your bacterial counts. One way to do this would be to repeat the counts multiple times (minimum of three replicates). Since the serial dilution process can introduce considerable error into the counts, you would need to do a separate serial dilution for each count. Then you can take the average of your counts, and the standard deviation will give you an estimate of your error.

You could expand the experiment to include different types of meat. Are some types of meat more prone to contamination than others?

What was the most important thing you learned?
I learned that it is a good idea to thaw your meat in the refrigerator (which I didn't know!) and cook it to the right temperature to prevent bacteria growth.

What problems did you encounter?
Some of these materials were hard to find.

Can you suggest any improvements or ideas?
Could have a more engaging "abstract" statement!

Overall, how would you rate the quality of this project?
Very Good

What is your enthusiasm for science after doing your project?

Compared to a typical science class, please tell us how much you learned doing this project.

Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

If you like this project, you might enjoy exploring these related careers:

Microbiologist

Microorganisms (bacteria, viruses, algae, and fungi) are the most common life-forms on Earth. They help us digest nutrients; make foods like yogurt, bread, and olives; and create antibiotics. Some microbes also cause diseases. Microbiologists study the growth, structure, development, and general characteristics of microorganisms to promote health, industry, and a basic understanding of cellular functions.
Read more

Agricultural Inspector

Who works to protect the public health from food-borne illnesses? Agricultural inspectors. Everyone needs to eat, and agricultural inspectors work to ensure the quality and safety of the food supply to determine if they are in compliance. They also inspect farms, businesses, and food-processing plants to determine if they are in compliance with government food regulations and laws.
Read more

Food Science Technician

Good taste, texture, quality, and safety are all very important in the food industry. Food science technicians test and catalog the physical and chemical properties of food to help ensure these aspects.
Read more

News Feed on This Topic

,
,

Note: A computerized matching algorithm suggests the above articles. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results! Learn more about the News Feed

Looking for more science fun?

Try one of our science activities for quick, anytime science explorations. The perfect thing to liven up a rainy day, school vacation, or moment of boredom.

Can you suggest any improvements or ideas?(Enter "no" if you have none.)

characters left

Overall, how would you rate the quality of this project?

Excellent
Very good
Good
OK
Poor

What is your enthusiasm for science after doing your project?

Very high
High
Moderate
Low
Very low

Compared to a typical science class, please tell us how much you learned doing this project.

Much more
More
About the same
Less
Much less

Optional:Attach a picture of your project (JPG, JPEG, GIF, PNG only)

Optional:Caption for picture

characters left

You can find this page online at: http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p015.shtml

You may print and distribute up to 200 copies of this document annually, at no charge, for personal and classroom educational use. When printing this document, you may NOT modify it in any way. For any other use, please contact Science Buddies.